Method for controlling the quality of the coolant for fuel cell systems

ABSTRACT

A method for controlling the quality of the coolant for fuel cell systems, it being proposed to measure and monitor the insulation resistance of the load circuit of the fuel cell system. It has become apparent that there is a functional, uniquely defined relationship between the measured insulation resistance and the electrical conductivity of the coolant so that a separate measurement of the electrical conductivity by means of corresponding sensor systems and evaluation means can be dispensed with in the present invention.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German Patent DE 101 02 247.6,filed Jan. 19, 2001, the disclosure of which is expressly incorporatedby reference herein.

The invention relates to a method for controlling the quality of thecoolant, such as cooling water, for fuel cell systems.

Such a method is known from EP 0 043 941 B1. In said application, theforeign ion content of water is continuously monitored using aconductivity measurement, the water being cooled to a temperaturebetween 0 and 5° C. before the measurement. It has been found that thereis a greatly increased relative conductivity (related to high-puritywater) in this temperature range in comparison with a measurement atambient temperature or at higher temperatures.

This known quality control is also applied in fuel cell systems in whichthe foreign ion concentration in the cooling water should not exceed aspecific value. For this purpose, a conductivity sensor measures theelectrical conductivity of the cooling water. The use of such aconductivity sensor is, however, associated with additional effort andcosts. At the temperatures which occur, there is also a poorsignal-to-noise ratio. Such prior cooling of the cooling water formeasuring the conductivity, as proposed in EP 0 043 941 B1, wouldfurther increase the costs and the effort.

The object of the present invention is therefore to specify a qualitycontrol for coolants such as cooling water for fuel cell systems, whichoperates without additional effort and costs, but with a high degree ofreliability.

This object is achieved according to the invention by means of a methodfor controlling coolant quality of a fuel cell system via measuring theinsulation resistance of the load circuit. In a preferred embodiment,the method comprises establishing a relationship between the electricalconductivity of the coolant and the insulation resistance of the loadcircuit; measuring the insulation resistance of the load circuit todetermine the electrical conductivity; and monitoring the electricalconductivity of the coolant. Further advantageous refinements will beevident from the following description.

According to the invention, the insulation resistance of the loadcircuit of the fuel cell system is measured and monitored. It hasspecifically been found that the insulation resistance has arelationship with the quality of the fuel cell coolant (cooling water)and thus with the electrical conductivity of this coolant. Because, forsafety reasons, the insulation of the load current lines in fuel cellsystems is generally monitored, conclusions about the coolant (coolingwater) quality of the fuel cell system can be drawn by means of theinvention from the monitoring of the insulation resistance withoutadditional effort.

A suitable method for monitoring the insulation resistance of a loadcircuit of a fuel cell system is disclosed in DE-195 03 749 C1,corresponding to U.S. Pat. No. 5,760,488, which is incorporated hereinby reference in its entirety. In said publication, it is proposed toimplement the fuel cell-fed or battery-fed power supply system of avehicle as an IT system (See Association of German ElectrotechnicalEngineers, Draft Standard Concerning DIN VDE 0100, Part 410, A3, Section413, June, 1989) in which the loads which are connected to the loadcircuit are electrically connected to the bodywork of the vehicle withlow impedance. The load circuit lines are preferably placedsymmetrically at a potential above or below the potential of thebodywork of the vehicle here and connected to the bodywork of thevehicle at high impedance. Damage (for example damage to the fuel cellstack or short-circuiting of the load circuit line to the vehiclebodywork) can be detected by means of a measuring bridge balancing stageand a measuring signal-conditioning isolating amplifier stage. The wayof implementing an insulation resistance monitoring system is describedin detail in the aforesaid document DE 195 03 749 C1. The following isrestricted to the details which are relevant to the present invention sothat for further details on the insulation resistance monitoring means,the aforesaid document is referred to expressly.

In an advantageous refinement of the invention, a lower threshold valueof the insulation resistance of the load circuit of the fuel cell systemis defined, and when the insulation resistance drops below this value,the replacement of the coolant which is due is indicated. Thisindication is given by means of a visual signal, an audible signal or acombined signal.

In a further refinement, a further threshold value of the insulationresistance which is preferably below the first-mentioned value isdefined, and when the insulation resistance drops below this thresholdvalue the entire fuel cell system is shut down. In this case, thequality of the coolant has degenerated drastically to the extent thatthe coolant (cooling water) is no longer suitable for further use in thefuel cell system. The associated threshold value of the insulationresistance can be greater than or equal to that threshold value at whichdamage has occurred as a result of short-circuiting, leakage currents ordamage to the fuel cell stack.

By making different selections of the threshold values of the insulationresistance for the quality control of the coolant, on the one hand, andfor the insulation control of the load current lines, on the other, itis possible to distinguish the type of fault or damage which occurs,i.e. different indications can be given for decreasing quality of thecooling water and for poor or missing insulation of the lines.

Furthermore, the insulation resistance and the electrical conductivityof the coolant (cooling water) in the fuel cell system can be measuredas a function of one another and the associated value of the electricalconductivity can be assigned to each measured value of the insulationresistance by reference to the relationship which is determined. Infact, it has become apparent that in a predefined system there is afixed relationship between the electrical conductivity of the coolingwater and the insulation resistance of the fuel cell system, thisrelationship behaving approximately as a 1/x function.

For individual fuel cell systems, or for a class of fuel cell systems,it is thus possible to determine a relationship between the insulationresistance and the electrical conductivity and to define a correspondingfunction. The upward transgression of the electrical conductivity abovespecific threshold values then corresponds to the downward transgressionof the insulation resistance below correspondingly different thresholdvalues. This transformation can consequently be carried out in a simpleway so that previous methods for quality control of the cooling water,which operate by measuring the electrical conductivity, can quickly betransformed to the method according to the invention.

An exemplary embodiment will explain the invention and its advantages inmore detail with reference to the appended FIGURE.

The single FIGURE shows the functional relationship between theinsulation resistance and the electrical conductivity in a fuel cellsystem.

In a fuel cell system for driving a motor vehicle, it is possible, asthe invention has shown, to determine the insulation resistance of theload circuit of the fuel cell system as a function of the electricalconductivity of the cooling water and to put it graphically in the formof the appended FIGURE. A person skilled in the art is sufficientlyfamiliar with the measurement of the electrical conductivity of thecooling fluid by means of a conductivity sensor. The measurement of theinsulation resistance is described in detail, as already mentioned, inDE 195 03 749 C1. In order to avoid repetitions, reference will be madeexpressly at this point to the content of this document.

It has become apparent that for various fuel cell systems of the sametype, a functional relationship between the insulation resistance andthe electrical conductivity can be derived, which relationship can berepresented by the following function

y=639.04 X ^(−0.7221)

given the dimensional relationships specified in the FIGURE.

The insulation resistance is given in the FIGURE in kOhm, and theelectrical conductivity in μS/cm. This functional relationshiprepresents, as is apparent from the FIGURE, a good approximation to theactual conditions (measuring points). A uniquely defined assignment ofthe electrical conductivity to corresponding values of the insulationresistance is thus possible. Consequently, known, upper threshold valuesfor electrical conductivity, above which cooling water is to be replacedor the system is even to be shut down, can be transformed intocorresponding, lower threshold values for the insulation resistance. Ifthe insulation resistance of the fuel cell system is monitored, which isgenerally the case for safety reasons, the measurement results can bere-used directly for the quality control of the coolant which is used,with the result that a separate sensor system with evaluation meansconnected downstream can be dispensed with.

The invention can be implemented particularly advantageously andcost-effectively in the fuel cell drives for vehicles and integratedinto existing systems.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed:
 1. A method for determining coolant quality of a fuelcell system which comprises a load circuit having an insulationresistance, the method comprising: determining the insulation resistanceof the load circuit; and determining said coolant quality as a functionof determined insulation resistance values.
 2. The method of claim 1,further comprising defining a first threshold value for the insulationresistance and signaling a need for the replacement of coolant when theinsulation resistance is below the first threshold value.
 3. The methodof claim 2, wherein the signaling is via at least one of a visual meansand an audio means.
 4. The method of claim 1, further comprisingdefining a second threshold value for the insulation resistance andshutting down the fuel cell system when the insulation resistance isbelow the second threshold value.